Photography for directly obtaining a positive image (direct positive) without requiring reversal processing or a negative film is well known in the art.
Conventionally known techniques for obtaining a positive from direct positive silver halide photographic materials, exclusive of special materials, are chiefly divided into the following two types.
A first technique employs a previously fogged silver halide emulsion whose fog centers (latent image) in exposed areas are destroyed, by solarization or the Herschel effect, to obtain a direct positive.
A second technique uses an internal latent image type silver halide emulsion that is unfogged, which is imagewise exposed to light and then subjected to surface development either after fogging or while fogging to obtain a direct positive. The internal latent image type silver halide emulsion used is an emulsion containing silver halide grains having sensitivity speck predominantly in the inside thereof, and forming a latent image predominantly in the grain interior upon exposure to light.
The latter materials generally have higher sensitivity and are suitable for uses requiring high sensitivity as compared with the former materials. The present invention relates to internal latent image materials.
Various techniques of this type have been proposed, such as those disclosed in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and British Pat. Nos. 1,151,363, 1,140,553 and 1,011,062. According to these conventional techniques, photographic materials providing a direct positive with relatively high sensitivity can be produced.
The details of the direct positive formation mechanism are disclosed, e.g., in T. H. James, The Theory of The Photographic Process, Chapter 7, pp. 182-193 (4th ed.) 1977 and U.S. Pat. No. 3,761,276. In greater detail, it is believed that a direct positive is formed through the following mechanism. First, imagewise exposure results in formation of an internal latent image ("positive hole") in the inside of silver halide grains, which leads to the formation of fog centers selectively on the surface of unexposed silver halide grains by surface desensitization ascribed to the positive hole, and subsequent surface development results in formation of a direct positive on the unexposed areas.
Selective formation of fog centers can be effected by a light fogging method in which the entire surface of a light-sensitive layer is secondarily exposed to light as described in British Pat. No. 1,151,363, or a chemical fogging method using a nucleating agent, as described in Research Disclosure, Vol. 151, RD No. 15162 (November, 1976), pp. 76-78.
In the formation of a direct positive image, the internal latent image type silver halide light-sensitive material is subjected to surface color development either after or simultaneously with fogging and is, if desired, further subjected to bleach and fixation (or blix). After the fixation, the material is usually washed with water and/or stabilized.
In direct positive formation by the above-described light fogging method or chemical fogging method, the rate of development is lower, requiring a longer development time, as compared with general negative working photographic materials. Hence, the pH and/or temperature of a developing solution used in these methods may be increased to thereby reduce the development time. However, use of a developing solution having a higher pH value generally involves the problem of an increase in minimum image density of the resulting direct positive. Further, the developing agent is easily deteriorated due to air oxidation under high pH conditions, so that the development activity is subject to variation.
Other known techniques for increasing the rate of development in direct positive formation include the use of hydroquinone derivatives as disclosed in U.S. Pat. No. 3,227,552, and the use of mercapto compounds having a carboxyl group or sulfo group as disclosed in Japanese Patent Application (OPI) No. 170843/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application"). However, these compounds produce only a small effect, and an effective technique for increasing a maximum density of a direct positive has not yet been discovered. In particular, technique has been sought for obtaining a sufficient maximum image density even when a developing solution at a low pH is employed.
In addition, the light fogging method presents various technical problems when applied to a broad photographic field for various purposes. Since this method is based on formation of fog centers by photolysis of silver halide, the optimum illumination or exposure varies depending on the kind and characteristics of the silver halide used. It is, therefore, difficult to assure consistent results. In addition, the development apparatus required is complicated and expensive, and the rate of development is also unsatisfactory.
When nucleation development is conducted in the presence of a nucleating agent by the chemical fogging method, conventionally known nucleating agents include quaternary heterocyclic compounds and hydrazine compounds. When the quaternary heterocyclic compound is used alone as a nucleating agent, efforts to obtain a high maximum density tend to be accompanied by an increase of the minimum density. Further, the maximum image density tends to decline due to insufficient stability under high temperature-high humidity conditions or high temperature-low humidity conditions. Furthermore, results are subject to great variation with pH change of the developing solution.
When using the hydrazine compound alone as a nucleating agent, sufficient maximum density cannot be obtained unless the pH of a developing solution is increased. As a result, the minimum image density increases as described above, and also the development activity of the developing solution is seriously reduced.
It has also been proposed to use two kinds of hydrazine compounds in combination, but this cannot solve the above-mentioned problem.
In the field of color developing solutions, various proposals having so far been made in an attempt to accelerating development and improve color formation. Considering that a color developing agent should penetrate into dispersed oil droplets of a coupler in order for the color developing agent to be finally coupled with a coupler to form a dye, various additives for accelerating the penetration of the color developing agent into the oil droplets of the coupler have been developed. Among the additives, benzyl alcohol is known to have a great accelerating effect on color formation and is now conventionally employed as an essential component of a processing solution for various color photographic materials.
Since benzyl alcohol has poor solubility, though soluble in water to some extent, it is commonly used in combination with solvents, such as diethylene glycol, triethylene glycol, and an alkanolamine, to improve its solubility. However, these compounds as well as benzyl alcohol itself are serious environmental pollutants which must be treated as a waste water as having high BOD and COD values. It is desirable, therefore, to reduce or eliminate benzyl alcohol from the standpoint of waste management despite the above-described advantages, such as improved color developability and improved solubility, which it provides.
Moreover, the use of cosolvents, e.g., diethylene glycol, remains insufficient to increase the solubility of benzyl alcohol to a desired level, while increasing the labor and time required for the preparation of a developing solution.
In addition, benzyl alcohol, when carried over into a bleaching bath or bleach-fix bath subsequent to the development and accumulated therein, is one cause of the formation of a leuco compound, depending on the type of a cyan dye used, which reduces color density. Further, the accumulation of benzyl alcohol hinders the washing off of developing solution components, and particularly of color developing agents, thus reducing image preservability due to the residual developing solution components.
From all these considerations, reduction or elimination of benzyl alcohol from a color developing solution is of great industrial significance.
Color photofinishing laboratories are confronted with not only the above-enumerated problems, but also the necessity of shortening the processing time to satisfy users' demands.
However, these requirements cannot be fulfilled simultaneously by any of the conventional techniques, since reduction of the development time without using benzyl alcohol in a color developing solution would seriously reduce the color density obtained.